1. Field of the Invention
The present invention relates to a technique for reading out information recorded on magnetic recording mediums and, more particularly, to a method of and apparatus for reproducing digital information recorded on such magnetic cards as entrance tickets, passenger tickets and lodging tickets which are not used as frequently or as repeatedly as commutation passes, credit cards or other essentially permanent cards.
Usually, magnetic cards for repeated and frequent use such as credit cards are rigid or coated on both faces with protective layers of a transparent plastic sheet, or other necessary measures are taken to prevent accidental bending of the magnetic surfaces, so that the information recorded thereon is not destroyed. Therefore, in these magnetic cards, destruction of information or information reading errors due to bending or distortion of the cards is avoided.
However, entrance tickets, passenger tickets or the like are usually of a thin construction from an economic view point because they are thrown away after the use. Thus, although it is prohibited to bend these cards, bending often occurs, resulting in the reading out and reproduction of erroneous information due to the destruction of the recorded information. This largely hinders the automation of various equipment and does not promote a reduction in labor.
The present invention aims at making it possible to reproduce correct information, even in the above-mentioned cases, by adopting a suitable interpolation technique.
The method and apparatus in accordance with the invention is effective particularly for less expensive magnetic cards in which the destruction of recorded information is likely to occur during handling. However, the invention can be applied not only to magnetic cards but also to other recording mediums including magnetic tape in which destruction of recorded information may take place.
2. Description of the Prior Art
A parity check system has been conventionally used, wherein parity bits are provided as means for checking the omission of bits due to the breakage of a recording or a similar reason, in the reproduction of information of the kind described.
More particularly, numerals are represented by bits in memory devices, and check bits are provided such that the sum of the number of bits magnetized in a plus direction always assumes an odd number. Thus during a data transfer, a check is conducted as to whether a number is an odd number or not; data which is not indicative of an odd number is detected as an error. This technique is referred to as odd number parity check, and has been widely used.
Meanwhile, a magnetic recording system relying upon magnetic cards, a binary recording method referred to as an NRZ-I method (Non-return to Zero and Invert at one or zero) is usually adopted, in which a magnetic inversion is effected by one datum, while the other datum does not cause the magnetic inversion.
In this NRZ-I method, a timing signal for defining the reading time is recorded in synchronization with the data, in addition to the parity bits. Thus, when the timing signal is missing and hence not reproduced due to bending of the magnetic surface, the character of the bit cell is judged invalid, even if the data can be read out, because the timing signal is missing. This NRZ-I method is disclosed, for example, in an article entitled "Spectrum Analysis of Digital Magnetic Recording Waveforms" by Arnold L. Knoll, inserted in IEEE Transactions on Electronic Computers, Vol. EC-16, No. 6 (December 1967), particularly in Table 1 and FIG. 1, page 733.
Also, the technique for reading out the magnetic information is disclosed, for example, in the specification of U.S. Pat. No. 3,840,892 (Oct. 8, 1974; Yukitaka Hayashi) Entitled "Method and Device for Detecting Signals from Magnetic Memory".
The Hayashi invention is concerned with a technique for reading out information by the NRZ-I method, and makes it possible to logically correct read out failure attributable to the lowering of the signal level due to the interference of the read out waveforms. This technique makes the most of the NRZ-I method, and performs a logical correction when two positive or negative pulses are successively detected, upon deciding that a signal is lacking between the two successive pulses.
The present invention is similar to the Hayashi invention in that it is adapted to decide that there is a missing signal when two positive or negative pulses are detected successively, as will be detailed later.
However, the Hayashi invention is effective, as will be seen from time charts of FIGS. 5 and 6, only in such a case where the data to be read out is lacking but where the clock pulses are normally available.
In practice, however, there are cases where the clock pulses themselves are omitted. In such a case, no interpolation of data can be made by the technique of the Hayashi invention.
The recording density on magnetic cards is usually much lower than that of other magnetic recording mediums such as magnetic tapes. Thus, in the case of the reproduction from magnetic cards, the reproduction is adversely affected by the omission of signals including clock pulses, rather than by the interference of the waveforms.
Unfortunately, however, there has not yet been proposed an effective measure, for correctly reading out data when there is a missing clock pulse.